The current invention is in the field of differential GPS.
In the available art, the Global Positioning System (GPS) is a system of satellite signal transmitters that transmits information from which an observer""s present location and/or the time of observation can be determined. Another satellite-based navigation system is called the Global Orbiting Navigational System (GLONASS), which can operate as an alternative or supplemental system.
The GPS was developed by the United States Department of Defense (DOD) under its NAVSTAR satellite program. A fully operational GPS includes more than 21 Earth orbiting satellites approximately uniformly dispersed around six circular orbits with four satellites each, the orbits being inclined at an angle of 55xc2x0 relative to the equator and being separated from each other by multiples of 60xc2x0 longitude. The orbits have radii of 26,560 kilometers and are approximately circular. The orbits are non-geosynchronous, with 0.5 sidereal day (11.967 hours) orbital time intervals, so that the satellites move with time relative to the Earth below. Generally, four or more GPS satellites will be visible from most points on the Earth""s surface, and can be used to determine an observer""s position anywhere on the Earth""s surface, 24 hours per day. Each satellite carries a cesium or rubidium atomic clock to provide timing information for the signals transmitted by the satellites. An internal clock correction is provided for each satellite clock.
Each GPS satellite continuously transmits two spread spectrum, L-band carrier signals: an L1 signal having a frequency f1=1575.42 MHZ (nineteen centimeter carrier wavelength) and an L2 signal having a frequency f2=1227.6 MHZ (twenty-four centimeter carrier wavelength). These two frequencies are integral multiplies f1=1,540 f0 and f2=1,200 f0 of a base frequency f0=1.023 MHZ. The deployment of additional frequencies is being planned by the DOD.
The L1 signal from each satellite is binary phase shift key (BPSK) modulated by two pseudo-random noise (PRN) codes in phase quadrature, designated as the C/A-code and P-code. The L2 signal from each satellite is BPSK modulated by only the P-code. The nature of these PRN codes is described below.
Use of PRN codes allows use of a plurality of GPS satellite signals for determining an observer""s position and for providing the navigation information.
A signal transmitted by a particular GPS satellite is selected by generating and matching, or correlating, the PRN code for that particular satellite. Some of the PRN codes are known and are generated or stored in GPS satellite signal receivers operated by users.
A first known PRN code for each GPS satellite, sometimes referred to as a precision code or P-code, is a relatively long, fine-grained code having an associated clock or chip rate of f0=10.23 MHZ. A second known PRN code for each GPS satellite, sometimes referred to as a clear/acquisition code or C/A-code, is intended to facilitate rapid satellite signal acquisition and hand-over to the P-code and is a relatively short, coarser-grained code having a clock or chip rate of f0=1.023 MHZ. The C/A-code for any GPS satellite has a length of 1023 chips or time increments before this code repeats. The full P-code has a length of 259 days, with each satellite transmitting a unique portion of the full P-code. The portion of P-code used for a given GPS satellite has a length of precisely one week (7.000 days) before this code portion repeats.
Accepted methods for generating the C/A-code and P-code are set forth in the document ICD-GPS-200: GPS Interface Control Document, ARINC Research, 1997, GPS Joint Program Office, which is incorporated by reference herein.
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite (which includes orbital information about the transmitting satellite within next several hours of transmission) and an almanac for all GPS satellites (which includes a less detailed orbital information about all other satellites). The transmitted satellite information also includes parameters providing corrections for ionospheric signal propagation delays (suitable for single frequency receivers) and for an offset time between satellite clock time and true GPS time. The navigational information is transmitted at a rate of 50 Baud.
A second satellite-based navigation system is the Global Orbiting Navigation Satellite System (GLONASS), placed in orbit by the former Soviet Union and now maintained by the Russian Republic. GLONASS uses 24 satellites, distributed approximately uniformly in three orbital planes of eight satellites each. Each orbital plane has a nominal inclination of 64.8xc2x0 relative to the equator, and the three orbital planes are separated from each other by multiples of 120xc2x0 longitude. The GLONASS satellites have circular orbits with a radii of about 25,510 kilometers and a satellite period of revolution of 8/17 of a sidereal day (11.26 hours). A GLONASS satellite and a GPS satellite will thus complete 17 and 16 revolutions, respectively, around the Earth every 8 days. The GLONASS system uses two carrier signals L1 and L2 with frequencies of f1=(1.602+9 k/16) GHz and f2=(1.246+7 k/16) GHz, where k(=1,2, . . . 24) is the channel or satellite number. These frequencies lie in two bands at 1.597-1.617 GHz (L1) and 1,240-1,260 GHz (L2). The L1 code is modulated by a C/A-code (chip rate=0.511 MHZ) and by a P-code (chip rate=5.11 MHZ). The L2 code is presently modulated only by the P-code. The GLONASS satellites also transmit navigational data at a rate of 50 Baud. Because the channel frequencies are distinguishable from each other, the P-code is the same, and the C/A-code is the same, for each satellite. The methods for receiving and demodulating the GLONASS signals are similar to the methods used for the GPS signals.
Reference to a Satellite Positioning System or SATPS herein refers to a Global Positioning System, to a Global Orbiting Navigation System, and to any other compatible satellite-based system that provides information by which an observer""s position and the time of observation can be determined, all of which meet the requirements of the present invention.
A Satellite Positioning System (SATPS), such as the Global Positioning System (GPS) or the Global Orbiting Navigation Satellite System (GLONASS), uses transmission of coded radio signals, with the structure described above, from a plurality of Earth-orbiting satellites. A SATPS antenna receives SATPS signals from a plurality (preferably four or more) of SATPS satellites and passes these signals to an SATPS signal receiver/processor, which (1) identifies the SATPS satellite source for each SATPS signal, (2) determines the time at which each identified SATPS signal arrives at the antenna, and (3) determines the present location of the SATPS satellites.
The range (ri) between the location of the i-th SATPS satellite and the SATPS receiver is equal to the speed of light c times (xcex94ti), wherein (xcex94ti) is the time difference between the SATPS receiver""s clock and the time indicated by the satellite when it transmitted the relevant phase. However, the SATPS receiver has an inexpensive quartz clock which is not synchronized with respect to the much more stable and precise atomic clocks carried on board the satellites. Consequently, the SATPS receiver estimates a pseudo-range (pri) (not a true range) to each satellite.
After the SATPS receiver determines the coordinates of the i-th SATPS satellite by demodulating the transmitted ephemeris parameters, the SATPS receiver can obtain the solution of the set of the simultaneous equations for its unknown coordinates (x0, y0, z0) and for unknown time bias error (cb). The SATPS receiver can also determine velocity of a moving platform.
The following discussion is applicable to any satellite navigational system, but is focused on GPS applications to be substantially specific.
Differential Global Positioning System (DGPS) is a technique that significantly improves both the accuracy and the integrity of the Global Positioning System (GPS). The most common version of DGPS requires high-quality GPS xe2x80x9creference receiversxe2x80x9d at known, surveyed locations. The reference station estimates the slowly varying error components of each satellite range measurement and forms a correction for each GPS satellite in view. This correction is broadcast to all DGPS users on a convenient communication link. Typical ranges for a local area differential GPS (LADGPS) station are up to 150 km. Within this operating range, the differential correction greatly improves accuracy for all users, regardless of whether selective availability (SA) is activated or is not. This improvement in the accuracy of the Global Positioning System (GPS) is possible because the largest GPS errors vary slowly with time and are strongly correlated over distance. DGPS also significantly improves the xe2x80x9cintegrityxe2x80x9d of GPS for all classes of users, because it reduces the probability that a GPS user would suffer from an unacceptable position error attributable to an undetected system fault. Expected accuracies with DGPS are within the range from 1 to 5 meters.
Most DGPS systems use a single reference station to develop a scalar correction to the code-phase measurement. If the correction is delivered within 10 seconds, and the user is within 1000 km, the user accuracy should be between 1 and 10 meters.
Network of reference stations can be used to form a vector correction for each satellite. This vector consists of individual corrections for the satellite clock, three components of satellite positioning error (or ephemeris), and parameters of an ionospheric delay model. The validity of this correction still decreases with increased latency or age of the correction. However, compared to a scalar correction, a vector correction is valid over much greater geographical areas. This concept is called wide area DGPS, or WADGPS. Such network can be used for continental or even world-hemisphere coverage, because it requires many fewer reference stations than a collection of independent systems with one reference station each, and because it requires less communication capacity than the equivalent network of LADGPS systems.
Users with very stringent accuracy requirements may be able to use a technique called carrier-phase DGPS or CDPGS. For CDGPS, the definition of long baseline is arbitrary, but usually refers to baseline lengths exceeding 20 km and up to 100 km. Lines in excess of 100 km may be referred to as very long baselines.
These users measure the phase of the GPS carrier relative to the carrier phase at a reference site; thus achieving range measurement precision that is a few percent of the carrier wavelength, typically about one centimeter. These GPS phase comparisons are used for vehicle attitude determination and also in survey applications, where the antennas are separated by tens of kilometers. If the antennas are fixed, then the survey is called static, and millimeter accuracies are possible, because long averaging times can be used to combat random noise. If the antennas are moving, then the survey is kinematic, and shorter time constants should be used with some degradation of accuracy.
The given above discussion can be found in xe2x80x9cGlobal Positioning System: Theory and Applicationsxe2x80x9d, Volume II, Chapter 1, by Bradford W. Parkinson and James J. Spilker Jr., published by the American Institute of Aeronautics and Astronautics, Inc. in 1996.
Summary of differential GPS concepts and accuracies is given in Table I.
There are two major difficulties with Long Baseline RTK (LBRTK).
(1) Processing in real-time the combined base and rover GPS measurements to yield the baseline vectors with sufficient accuracyxe2x80x94which implies fixed integer multi-frequency solutions; and
(2) broadcasting the base (or reference) station GPS data to the roving station (rover), for example using a protocol such as the Trimble Compact Measurement record (Trimble CMR) data format, that was described in the paper xe2x80x9cCompact Data Transmission Standard for High-Precision GPSxe2x80x9d given by Dr. Nicholas C. Talbot at The Proceedings of the IX-th International Technical Meeting of the Satellite Division of the Institute of Navigation in the Kansas City, Mo., Sep. 13-20, 1996.
The first problem arises because atmospheric refraction of the satellite signals which has different magnitudes at the two stations makes processing the data over a long baseline with high accuracy very difficult. There are various ways to reduce these effects and increase baseline accuracy. For instance, the errors caused by ionospheric refraction can be reduced by combining satellite signals at two or more distinct frequencies and forming ionospheric-free measurements, while the errors caused by tropospheric refraction can be reduced by using a tropospheric model which can take into account the differences in the height between the two stations. Thus, despite the inherent errors cause by signals refraction, it is possible to compute accurate long baselines.
However, the second problem persists. Indeed, the problem of broadcasting the base station GPS data is difficult to solve due to radio licensing restrictions. The available bands in the frequency spectrum, the transmit bandwidth at specific frequencies, and the transmit power are regulated by agencies (for instance, FCC in the USA). Although, it is possible to obtain a license, it may not be possible to guarantee the availability of a clear channel for continuous data transmission. A conventional RTK system requires a clear channel for it operation. If the base station data cannot be received, the rover cannot compute relative positions in real time. Problems also occur due to the distance and the available transmit power. The signal may not have sufficient power to be received, or may be attenuated if there is no clear line of sight between the base and rover. Topology or foliage may block the signal entirely, depending on the transmit frequency.
Methods other than direct wireless links, for example, the cellular telephone, can be used between the base and rover. However, a continuous link may not be guaranteed if the rover moves between cells in the network. If cellular links are being used, they also suffer from data latency (a delay in the data that in turn delays computation of the position of the rover), and from high telephone connection charges.
What is needed is a system and method for LBRTK that (1) utilizes a non-continuous link between the base and rover, and (2) minimizes high connection charges by minimizing the amount of data needed to be transmitted from the base to the rover.
To address the shortcomings of the available art, the present invention provides, a system and method for LBRTK surveying using a non-continuous data link between a primary base station and a secondary base station (the significant portion of the baseline length), together with a secondary base station and a much shorter continuous one-way secondary data link to the rover. Although the system requires a total of three satellite receivers, it does not suffer from the complications inherent in network RTK system which typically require four or more receivers (three or more of which comprise the network which should be placed at known locations). The secondary base can be placed at an arbitrary location, for example at an elevated site providing clear reception of the secondary base from the rover. This elevated site may also provide clear communication between the primary and secondary bases if this data link is optionally a direct radio link. The primary base can be used to service a number of secondary bases at different survey sites.
One aspect of the present invention is directed to a system for long baseline real time kinematic positioning (LBRTK).
In one embodiment, the system comprises: (1) a primary base station (PBS); (2) a secondary base station (SBS); (3) a primary data link between the SBS and the PBS; and (4) a secondary data link between the SBS and a rover.
In one embodiment, the PBS is configured to transmit data to the SBS using the primary data link, the SBS is configured to compute a secondary base position (SBS_P) relative to a primary base position (PBS_P) using the transmitted data from the PBS; and the rover is configured to perform an RTK survey using the SBS_P transmitted to the rover using the secondary data link.
In one embodiment, the primary data link further comprises a primary two-way data link. The primary two-way data link can include: (1) a cellular telephone link; (2) a radio link; (3) an electronic mail link; or (4) a satellite link.
In another embodiment, the primary data link further comprises a primary one-way data link. In this embodiment, the PBS is configured to transmit a compressed data set to the SBS according to a predetermined schedule using the primary one-way data link.
In one embodiment, the secondary data link further comprises a secondary one-way data link. The secondary one-way data link can include: (1) a radio link; or (2) an optical link.
Another aspect of the present invention is directed to a system for long baseline real time kinematic positioning (LBRTK) for a plurality of rovers.
In one embodiment, the system comprises: (1) a primary base station (PBS); (2) a plurality of secondary base stations (SBS); (3) a plurality of primary data links between the SBS and each. PBS; and (4) a plurality of secondary data links. Each SBS and each rover are linked by at least one secondary data link.
One more aspect of the present invention is directed to a method for long baseline real time kinematic positioning (LBRTK).
In one embodiment, the method comprises the following steps: (a) receiving a first plurality of broadcast satellite signals by utilizing a primary base station (PBS) comprising a primary base multi-frequency satellite antenna; (b) continuously obtaining a first plurality of satellite observables using a primary base satellite receiver configured to utilize the first plurality of received satellite signals; (c) logging each satellite observable obtained from the first plurality of received satellite signals at a first predetermined interval using a primary base data storage; (d) receiving a second plurality of broadcast satellite signals by using a secondary base station (SBS) comprising a secondary base multi-frequency Satellite antenna; (e) continuously obtaining a second plurality of satellite observables using the second plurality of received satellite signals by utilizing a secondary base satellite receiver; (f) logging each satellite observable obtained from the second plurality of received satellite signals at a second predetermined interval by utilizing a secondary base data storage; (g) transmitting the PBS logged data from the PBS to the SBS using a primary data link; (h) computing a secondary base position (SBS_P) relative to a primary base position (PBS_P) using the transmitted PBS data; and (i) performing an RTK survey by a rover, wherein the rover is configured to receive the SBS_P transmitted to the rover using a secondary data link between the SBS and the rover.
Yet, one more aspect of the present invention is directed to a method of Long Base RTK (LBRTK) surveying of a plurality of survey marks employing a rover, a primary base station (PBS), and a secondary base station (SBS), wherein the SBS and the PBS comprise a long baseline.
In one embodiment, the method comprising the following steps: (a) setting up the PBS in a location having an known position; (b) setting up the SBS in a location having an unknown position, wherein the SBS location is set to provide a secondary communication link to the rover; (c) starting the PBS to automatically log a first plurality of satellite observables at a first predetermined rate; (d) storing the continuously logged PBS data in a primary database storage; (e) starting the SBS to automatically log a second plurality of satellite observables at a second predetermined rate; (f) storing the continuously logged SBS data in a secondary database storage; (g) initiating by the SBS a primary two-way communication link between the PBS and the SBS; (h) sending a request for a first PBS data transfer by the SBS to the PBS, wherein the requested first PBS data comprises PBS data logged during a time period between a time instance when the SBS was turned on and a time instance when the first sending request was sent; (i) accessing the primary PBS database storage, compressing the requested first PBS data, and sending the first requested compressed PBS data set to the SBS using the primary two-way communication link; (k) matching the first logged SBS data with the first transmitted compressed PBS data set; (l) computing an initial secondary base position (SBS_IP) relative to a primary base station position (PBS_P) by utilizing the first transmitted compressed PBS data set and the first logged matched SBS data; (m) continuously transmitting from the SBS to the rover initial data using the secondary communication link, wherein the initial data includes the SBS_IP; and (n) performing RTK surveying of each survey mark by the rover.